The present invention relates to color image forming devices such as digital color copiers. In particular, the present invention relates to a chroma control method that provides easy control for producing an output picture of a desired tone.
In a digital color copier, the image of the original is separated into three color components, red, green and blue, which are then transformed to colorant components, yellow, magenta and cyan, by matrix operations. Based on these trichromatic colorant components, a black colorant component for "black printing" is generated and eventually a yellow, a magenta, a cyan and a black colorant are deposited on receiving paper in amounts that are associated with the image of the original, thus producing a color copy.
Production of color copies involves more complicated optical and electrical processing than with monochromatic copies. Furthermore, any deviation from the original will be readily perceived by the eye, so that it is quite difficult to achieve faithful reproduction of the image of the original unless proper control is made. The situation is much more complicated since there are occasional needs to make copies with the tone of the original being deliberately altered. To meet this need, color copiers are furnished with special circuitry for controlling such parameters as density, saturation (chroma) and hue by altering the quantities, proportions and other variables of three color components (red, green and blue) or three colorant components (yellow, magenta and cyan).
A method for controlling chroma is described in the specification of Japanese Patent Application Unexamined Publication No. 61-179697. The method starts with determining the densities of three colors, red (D.sub.R), green (D.sub.G) and blue (D.sub.B), and then determining their respective maximum, intermediate and minimum values, MAX, MID and MIN, which are expressed by: EQU MAX=max(D.sub.R, D.sub.G, D.sub.B) EQU MID=mid(D.sub.R, D.sub.G, D.sub.B) EQU MIN=min(D.sub.R, D.sub.G, D.sub.B).
The thrust of this method lies in controlling chroma by altering the values of MAX and MID without changing the ratio of (MID-MIN)/(MAX-MIN).
This method, however, has had the problem that not only chroma but also lightness will vary since the densities of two colors are changed with the minimum density of the third color fixed. In order to avoid the variation in lightness, the value of MIN must also be changed but then the circuitry becomes complicated.
A method has also been employed to control chroma by directly altering the respective color components but in this method, too, unavoidable changes will occur not only in chroma but also in density and hue, thereby making it difficult to produce image of the intended tone. In another practice, various correction circuits are employed to insure that chroma control can be performed without influencing density and hue, but the points that need adjustment are so many that not only it is difficult to perform adjusting operations but also many years of experience are required to attain good results.